live conductors. Insulation oil (traditionally, mineral insulating oil) is used preliminarily for insulation of the core. The main functions of the insulation oil in an oil‐filled transformer are as follows:
Oil acts as an insulating medium: Provides insulation for the conductors.
Oil serves as a coolant by dissipating internally generated heat: Heat dissipation is done in conduction, convection, and radiation modes with the help of solid insulation and cooling tubes.
Oil acts as a diagnostic medium for prognosis of aging characterization of in‐service oil‐filled transformers: Small volume of the liquid is collected from the transformer to test according to the standards for diagnosis and prognosis in understanding the situations prevailing in the transformer (similar to blood in human body).
Oil acts as a barrier to the laminated sheet steel core by preventing direct contact with atmospheric air.
Mineral insulating oils extracted from crude petroleum are being used in transformers successively since decades. However, due to various technical demands/benefits, health/safety aspects, and environmental concerns, alternatives for mineral oils are of high demand by the industry. There is a need to shift the transformer insulation technology to a suitable new alternative and biodegradable liquids. Ester‐based dielectric fluids, both natural and synthetic fluids, have been subjected to extensive research since decades. The performance of these new insulating liquids is found to be affirmative and comparable to mineral insulation oils. Despite their overwhelming technical benefits, very few utilities started using these alternative insulating fluids. One of the reasons includes the availability of limited diagnostic and prognostic information in the existing knowledge. The reclamation aspects and service experience of these new insulating liquids also remain as a pertinent challenge for the utilities.
Meanwhile, there are some significant aspects of ester fluids that need to be further investigated to improve the existing knowledge. This book aims at discussing the critical aspects that will act as a guide to the industry and researchers. The critical concepts with the significant research findings over the years and the topics that will need to be further emphasized will be digested in the chapters of this book. The knowledge embodied in this book will also act as a quick reference for utility engineers, transformer owners, academia, and researchers those interested in alternative and biodegradable insulating fluids for liquid‐filled transformers. The scope of this book will aid to the focus of the IEEE DEIS technical committee on liquid dielectrics and lies within the interests of the IEEE DEIS community and transformer industry across the world. This book will be a unique sample of its kind and will act as a reference pertaining to the biodegradable insulating liquids. The contents of this book are framed in a way to cover the pertinent orbits of ester liquids, key aspects that need to be emphasized to further expand the existing knowledge, and act as a quick guide to the utility engineers. This book also covers the discussions on critical challenges that will be helpful for the engineers in successful operation of the ester‐filled transformers. Importantly, reputed and experienced researchers who are working on the individual subtopics for a long time contribute the chapters of this book. This will allow widening the scope and technical orbits of the book in an effective and unique manner. This book will also act in bridging the gap between the researchers and utility engineers concerning the transformer insulation systems.
1.2 Insulation System in Liquid‐Filled Transformers
Liquid insulation in transformers plays a critical role in assessing the lifetime of a transformer. Useful life of a transformer relies on the electrical health and effectiveness of insulating oil. Owing to prevalent thermal conditions within an in‐service oil‐filled transformer, the performance of solid insulation paper, pressboard, etc., and the effectiveness of the insulating oil to serve as an insulator and as a coolant, reduces significantly over a period. High thermal excursions in the transformer tend to accelerate the aging process of the oil–paper insulation. Eventually, insulation paper degrades by releasing certain gases, moisture, furan‐based compounds, and suspended particles percolating into the insulation oil; thus, enhances the deterioration of oil. As per ASTM D117‐18 [20] standard, commonly adapted parameters of insulation oil to monitor the transformers are classified as electrical, physical, and chemical.
Hence, degradation of insulation paper can be estimated by performing diagnostic and prognostic tests on the insulation oil. Simply, characterization of insulation oil facilitates prediction of performance aspects of insulation and helps estimate the prevailing conditions within the in‐service oil‐filled transformers. It has been a past practice of the power industry to perform diagnosis tests on the insulation oil in view of condition monitoring of an oil‐filled transformer. Insulation system being the heart of the transformer comprises the liquid insulation (typically oils) and solid insulation system (typically cellulose). Traditionally, mineral insulating oils are put up in practice for the transformer insulation technology. Due to the fact that mineral oils are expected to reach depletion, alternative candidates for these oils is a challenge. Further, mineral oils are toxic and nonbiodegradable. Thus, demanding an alternative candidate, which is biodegradable and nontoxic while meeting the technical requirements that an insulating liquid should exhibit. It is desired that a transformer insulating liquid has good dielectric strength, high thermal performance (high fire point), and has good compatibility with the transformer solid insulation. Classification of several insulation parameters that are used by the industry to monitor the performance of transformer insulation is shown in Figure 1.1.
Parameters highlighted in Figure 1.1 are fundamental parameters that majority of the utilities monitor to access the aging level of insulation in the transformers. All the abovementioned parameters of in‐service insulation oil are classified based on the nature of liquid insulating medium (property) in oil‐filled transformers. However, the aging aspects of transformers are highly interrelated and measuring oil–paper insulation aging has become an established interdisciplinary engineering application field of materials science and chemical engineering through latest instrumentation and aging measurements by various techniques and processes.
Figure 1.1 Basic insulation parameters for aging assessment of oil–paper insulation.
1.3 Insulation Aging Phenomena in Transformers
The service life of the oil‐filled apparatus is mainly attributable to the rate of degradation of the oil/paper insulation system. The preliminary causes for the degradation of the oil/paper insulation system are its decomposition aspects. The degradation process of the insulation system in oil‐filled apparatus is governed by the: (i) heat (dissipated through the core and winding assembly), (ii) moisture (ingressed from the air and liberated from cellulose), and (iii) oxygen (ingressed from air and liberated from oil/paper system). This operating heat will adversely affect the longevity and performance of the insulation system and the life of the apparatus if not dissipated properly in time [21]. As per IEC 60076‐2 standard (thermal layout), insulation paper will serve for up to 55 years and more, subject to the absence of dielectric defects in the insulation system. Albeit due care is taken by the manufacturers in designing, the generation of heat from coil–core assembly and subsequent degradation of the insulation system is unavoidable. The degradation of oil/paper insulation generates soluble and colloidal particles in the oil. This increases the oil viscosity, which hampers the flow rate in cooling tubes and affects the heat dissipation efficiency. This excess heat is responsible for the accumulation of hot spots within the transformer, which further intensifies the degradation mechanisms. The degradation mechanisms of an oil/paper insulation system include
